The invention relates to a process for the direct reduction of particulate iron-oxide-containing material by fluidization, wherein synthesis gas such as reformed natural gas is introduced as a reducing gas into several fluidized bed zones consecutively arranged in series for the reducing gas and is conducted from one fluidized bed zone to another fluidized bed zone in counterflow to the particulate iron-oxide containing material, and wherein heating of the iron-oxide-containing material is effected in the fluidized bed zone arranged first in the flow direction of the iron-oxide-containing material and direct reduction is carried out in the further fluidized bed zone(s), as well as a plant for carrying out the process.
A process of this kind is known from U.S. Pat. No. 5,082,251, WO-A-92/02458 and EP-A-571 358. According to U.S. Pat. No. 5,082,251, iron-rich fine ore is reduced in a system of fluidized bed reactors arranged in series by aid of a reducing gas under elevated pressure. The thus produced iron powder is then subjected to hot or cold briquetting.
The reducing gas is produced by catalytic reformation of desulfurized and preheated natural gas with superheated water vapor in a conventional reformer furnace. After this, the reformed gas is cooled in a heat exchanger and, subsequently, the H2 portion in the reducing gas is increased by CO conversion by aid of an iron oxide catalyst. After this, the CO2 forming as well as the CO2 coming from the reformer are eliminated in a CO2 scrubber.
This gas is mixed with the reducing gas (top gas) consumed only partially, heated and the fine ore is reduced in three steps (three fluidized bed reactors) in counterflow.
The ore flow starts with drying and subsequent screening. Then the ore is transferred into a preheating reactor in which natural gas is burnt. In three consecutive reactors, the fine ore is reduced under elevated pressure.
From EP-A 0 571 358 it is known to realize the reduction of fine ore not exclusively via the strongly endothermic reaction with H2 according to
Fe2O3+3H2=2Fe+3H2xe2x88x92xcex94H, 
additionally via the reaction with CO according to
Fe2O3+3CO=2Fe+3CO2xe2x88x92xcex94H, 
which is exothermic. It is thereby feasible to considerably lower operating costs and, in particular, energy costs.
According to the prior art, direct reduction, because of the kinetics of the known processes, involves magnetite formation during direct reduction in a layer constantly growing from outside towards inside and forming on each particle or grain of the iron-oxide-containing material. It has been shown in practice that the formation of magnetite has an inhibiting effect on direct reduction with a reducing gas. Thus, it is feasible only at elevated expenditures, i.e., by increasing the consumption of reducing gas, to obtain a more or less complete reduction of the iron-oxide-containing material charged. In particular, it is necessary to make available a reducing gas having a high reduction potential even in the fluidized bed zones arranged first.
The invention aims at avoiding these disadvantages and difficulties and has as its object to further develop a process of the initially defined kind with a view to lowering the energy demand by fully utilizing the chemical potential of the reducing gas. In particular, operating costs are to be considerably lowered by utilizing the reducing gas to an optimum degree both in terms of reduction potential and in terms of sensible heat.
In accordance with the invention, this object is achieved
in that a temperature of the iron-oxide-containing material of either below 400xc2x0 C. and, preferably, below 350xc2x0 C.
or above 580xc2x0 C. and, preferably about 650xc2x0 C.,
or a temperature ranging from 400 to 580xc2x0 C. is adjusted in the first fluidized bed zone,
wherein, at a temperature adjustment to below 400xc2x0 C., the temperature range between 400xc2x0 C. and 580xc2x0 C. in the fluidized bed zone following the first fluidized bed zone in the flow direction of the iron-oxide-containing material is passed through within a period of 10 minutes and, preferably, within 5 minutes, and
wherein, at a temperature adjustment to above 580xc2x0 C., the temperature range between 400xc2x0 C. and 580xc2x0 C. is passed through within a period of maximally 10 minutes and, preferably, 5 minutes, and
wherein, furthermore, at a temperature adjustment in the range of from 400xc2x0 C. to 580xc2x0 C., the iron-oxide-containing material remains within that temperature range for a maximum of 10 minutes and, preferably, 5 minutes and is passed on into the fluidized bed zone following next immediately after having reached the desired temperature.
By these measures, it is feasible to effectively avoid or reduce to an acceptable extent, the formation of magnetite layers. The formation of a magnetite layer occurs very rapidly, i.e., the more rapidly the closer the temperature of the iron-oxide-containing material approaches to the limit temperature of about 580xc2x0 C. A magnetite formed on the surface of a particle of iron-oxide-containing material or an ore grain is denser than the ore itself, thus increasing the diffusion resistance of the interface between reducing gas and iron ore. As a result, the reaction speed is reduced. According to the Baur-Glaessner diagram, such a formation of a dense magnetite layer on the surface of an iron ore grain primarily occurs up to a temperature of the iron ore of 580xc2x0 C. At a temperature of the iron ore below 400xc2x0 C., the formation of magnetite is again slowed down and, as a result, dense magnetite layers are formed less rapidly.
The reaction kinetics of magnetite formation is influenced by the composition of the gas and of the solid. The molecules of the reducing gas must travel from the outer gas flow through the adhering gas border layer and through the macropores and micropores to the site of reaction. There, the dissociation of oxygen takes place. The oxidized gas returns via the same manner. The ore grains is, thus, reduced from the outside towards the inside. Thereby, its porosity increases, since the dissociated oxygen leaves behind hollow spaces and the original volume of the ore grain does not shrink significantly. The reaction front migrates from the outside towards the inside into the ore grain. With dense layers, the concentration of the reducing gas decreases from the outside towards the inside. The gas at first diffuses from the outside through the already reduced shell as far as the to the reaction front, where it is reacted and then diffuses back as a reaction product. With porous surfaces, the phase border reaction occurs on the walls of the pores within the reaction front, while the gas at the same time also may diffuse inside. With dense magnetite layers on the surface of the ore grain, the reaction kinetics is inhibited because the reducing gas is impeded from diffusing by the magnetite layer and the mass transfer of the reducing gas thus cannot occur in the same manner as with porous grains.
The basic idea of the invention is to be seen in accomplishing the transition of the temperature of the iron-oxide-containing material during heating from 400 to 580xc2x0 C. within as short a period of time as possible and avoiding maintenance within that critical temperature range. When rapidly passing that temperature range, the formation of a magnetite layer is extremely modest. If at all, wuestite is formed, which is not disadvantageous to reduction. Hence result substantially enhanced reduction conditions for the fluidized bed zone arranged first in the flow direction of the iron-oxide-containing material.
Advantageously, the iron-oxide-containing material in any event is transferred to the consecutively arranged fluidized bed zone immediately after having reached the desired temperature.
According to a preferred embodiment, the temperature range between 400xc2x0 C. and 580xc2x0 C. is passed through while avoiding a residence time, the average temperature gradient within the range of between 400xc2x0 C. and 580xc2x0 C. amounting to at least 20xc2x0 C./min and, preferably, 40xc2x0 C./min.
If, in that first fluidized bed zone, a temperature of but below 400xc2x0 C. is adjusted, the temperature range between 400 and 580xc2x0 C. will be passed in the fluidized bed zone arranged second in the flow direction of the iron-oxide-containing material, there occurring at a substantially higher speed than would be possible under normal conditions in the first fluidized bed zone, since the temperature of the reducing gas in the second fluidized bed zone is still substantially higher and, in addition, the reduction potential is higher, too. The latter likewise impedes or reduces the formation of magnetite. In that case, the rapid passage through the critical temperature range takes place in the second fluidized bed zone also within a noncritical period of time.
If the critical temperature range is to be passed through only in the second fluidized bed zone, this may be effected in various ways.
Thus, this may, for instance, be reached in that the reducing gas fed to the first fluidized bed zone is subjected to cooling before being introduced into the first fluidized bed zone.
An effective temperature adjustment to below 400xc2x0 C. in the first fluidized bed zone may also be obtained in that the reducing gas emerging from the fluidized bed zone arranged to follow the first fluidized bed zone in the flow direction of the iron-oxide-containing material is introduced into the first fluidized bed zone only partially and the reducing gas emerging from the first fluidized bed zone is recirculated into the first fluidized bed zone at least partially.
According to a preferred embodiment, the iron-oxide-containing material and the gas are indirectly cooled in the first fluidized bed zone, preferably by means of air or water.
It is also possible to directly cool the iron-oxide-containing material and the gas in the first fluidized bed zone, preferably by nozzling in water and/or water vapor.
According to a variant to be carried out in a particularly simple manner, maintaining the iron-oxide-containing material under the critical temperature in the first fluidized bed zone is ensured in that the iron-oxide-containing material has a shorter residence time in the first fluidized bed zone than in the following fluidized bed zones consecutively arranged in the flow direction of the iron-oxide-containing material.
This may be achieved in that the reducing gas fed to the first fluidized bed zone is heated as a total or partial stream, preferably indirectly by means of a smoke gas, before being introduced into the first fluidized bed zone.
According to a preferred variant, the invention is realized in that the total amount, or only a partial amount, of the reducing gas emerging from the fluidized bed zone arranged to follow the first fluidized bed zone in the flow direction of the iron-oxide-containing material and at least a partial amount of a fresh and, preferably, unused reducing gas are fed into the first fluidized bed zone.
Another preferred embodiment is characterized in that oxygen or an oxygen-containing gas is supplied to the reducing gas fed to the first and/or consecutively arranged fluidized bed zone while effecting a partial combustion of the reducing gas, prior to its entry into the first fluidized bed zone.
A further variant is characterized in that oxygen or an oxygen-containing gas is introduced into the first and/or consecutively arranged fluidized bed zone while effecting a partial combustion of the reducing gas.
The critical temperature range also may be rapidly passed through in that the iron-oxide-containing material charged into the first fluidized bed zone is charged in the preheated state, preferably in the highly preheated state, and, in particular, at a temperature ranging above 250xc2x0 C.
A further preferred embodiment is characterized in that the iron-oxide-containing material and the gas are indirectly heated in the first fluidized bed zone, preferably by means of a hot gas or by means of a smoke gas or by burning a burning gas.
It goes without saying that the object of the invention may be achieved also by applying two or several of the above-described variants in combination.
A plant according to the invention for carrying out the processes of the invention comprising several fluidized bed reactors consecutively arranged in series for receiving an iron-oxide-containing material with the iron-oxide-material being conducted from one fluidized bed reactor to another fluidized bed reactor via conveying ducts in one direction and the reducing gas being conducted from one fluidized bed reactor to another fluidized bed reactor via connecting ducts in the opposite direction is characterized in that a recuperator is provided in the fluidized bed reactor arranged first in the flow direction of the iron-oxide-containing material.
Plants for carrying out the processes according to the invention are defined in the subclaims. Such plants are partially known per se, for instance, from EP-A-0 571 358 (adjustment of an elevated temperature in the first reduction reactor). In addition, from U.S. Pat. No. 3,205,066 the partial combustion with oxygen or an oxygen-containing gas in a fluidized bed, from U.S. Pat. No. 3,982,901 and U.S. Pat. No. 3,983,927 the installation of heat exchangers in fluidized bed reactors, and from EP-A-0 345 467 the provision of jacketed jet heating pipes in fluidized bed reactors are known per se.